The basic implantable cardioverter/defibrillator or pacemaker system consists of at least one electrode attached to the heart and connected by a flexible lead to a pulse generator. This generator is a combination of a power source and the microelectronics required for the system to perform its intended function. A fixed rate pacemaker provides continuous pulses to the heart, irrespective of proper heart beating, while a demand inhibited pacemaker provides pulses only when the heart fails to deliver a natural pulse. Depending upon the various sensed events, the pacemaker stimulates the right atrium, the right ventricle, or both chambers of the heart in succession. The pacemakers in current use incorporate circuits and antennae to communicate non-invasively with external instruments called programmers. Most of today's pacemakers are of the demand inhibited type, hermetically sealed, and programmable. Implantable cardioverters/defibrillators add the capability of correcting dangerous arrhythmias and fibrillation by selected stimulation patterns or high energy shocks. High energy shocks are used primarily to correct life-threatening fibrillations by essentially stopping the heart and allowing an appropriate rhythm to re-establish itself.
To achieve reversion to an organized rhythm, it is generally believed that the heart should receive an effective amount of electrical energy, equal to or exceeding a threshold energy. The threshold energy may vary from patient to patient. Consequently, a physician will customarily perform certain tests when implanting a defibrillator and will select an energy level for defibrillating shocks. Implantable defibrillators are configured to deliver electrical energy by controlling the voltage on or applied from output capacitors. The energy delivered to the heart is often set on the assumption that the impedance of the electrical path including the leads and the heart is fixed.
Unfortunately, a fixed impedance is not assured at implant and its characteristics over time cannot be guaranteed. There may be changes in the heart itself, or in surrounding tissue. The position of the leads may change, or the body may react to the presence of the leads as foreign bodies. If the impedance of the electrical path changes, the amount of electrical energy delivered to the heart will also change, and may unintentionally fall below the threshold energy or the energy level prescribed by the attending physician.